In the discussion that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art against the present invention.
In order to facilitate the understanding of the invention, reference is made by way of introduction to accompanying FIGS. 1-4, FIG. 1 of which illustrates a turning tool during conventional machining of a workpiece, while FIGS. 2-4 schematically illustrate different conditions for the guidance of the removed chip.
In FIG. 1, a turning tool 1 is shown during machining of a workpiece 2. The tool 1 includes a holder 3 as well as a turning insert 4 (made in accordance with the invention). In this case, the workpiece 2 is rotated (in the direction of rotation R) at the same time as the tool 1 is longitudinally fed parallel to the centre axis C of the workpiece, more precisely in the direction of arrow F. The longitudinal feed per revolution is designated f, while the cutting depth is designated ap. The setting angle between the direction of the longitudinal feed and a main edge included in the turning insert is designated κ. In the example shown, κ amounts to 95°. It should furthermore be pointed out that the turning insert 4 has a rhombic basic shape and comprises two acute-angled corners having an angle of 80° and two obtuse-angled ones having an angle of 100°. In such a way, a clearance angle σ of 5° is obtained between the turning insert and the generated surface of the workpiece.
In FIGS. 2-4, CE designates a cutting edge that has a positive cutting geometry and is delimited between a chip surface CS and a clearance surface CLS. The surfaces CS and CLS meet each other at an acute angle, and therefore the rake angle RA of the cutting edge becomes smaller than 90°. In the example, RA amounts to approximately 15°. An upper side SS useful as a bearing surface transforms, via a boundary line BL, into a flank surface FS, which leans downward toward a bottom B, which forms a transition to the chip surface CS. The distance between the boundary line BL and the cutting edge line of the cutting edge CE is designated L. A chip removed by the cutting edge CE is illustrated schematically in the form of an arc line CH.
In all kinds of chip removing machining of metal, including turning, the rule applies that the chip is born “crooked”, i.e., immediately after the moment of removal, the chip obtains an inherent aim to be curved. The shape of the chip, among other things its radius of curvature thereof, is determined by several factors, the most important of which in connection with turning are the feeding of the tool, the rake angle of the cutting edge, and the cutting depth in question. After the removal, the chip will move perpendicularly to each infinitesimal part of the cutting edge. If the cutting edge is straight, the chip therefore becomes cross-sectionally flat or rectangular, but if the same is entirely or partly arched, also the chip becomes cross-sectionally entirely or partly arched.
In FIG. 2, it is shown how the chip CH is formed without hitting the flank surface FS. This means that the chip is developed in an uncontrolled way without being guided at all. Such a chip most often curls into a long, telephone cord-like screw formation, which among other things may hit the generated surface of the workpiece, get entangled in the tool and/or the machine, and at times even be a risk of damage to the surroundings. In the example according to FIG. 2, the level difference H1 between the bearing surface SS and the cutting edge CE—or the height of the flank surface FS above the cutting edge CE—in relation to the distance L is too small for the chip to contact the flank surface FS. It may also be said that the flank surface FS is situated at too great a distance from the cutting edge CE to be hit by the chip having the radius of curvature in question.
In FIG. 3, a turning insert is shown, in which the level difference H2 between the bearing surface SS and the cutting edge CE (=the height of the flank surface) is considerably greater than in the preceding example, the flank surface FS sloping fairly steeply down toward the transition B to the chip surface CS. This means that the chip CH will dive into the flank surface FS with a great force, more precisely in a lower area of the same. The result of this will be that great amounts of heat are developed in the contact area, at the same time as the turning insert becomes blunt-cutting. In addition, the material of the chip may easily stick to the flank surface FS, even all the way up to the bearing surface SS. After a certain time of use, also wear damage in the flank surface arises. Therefore, the embodiment according to FIG. 3 does not provide good chip control.
In FIG. 4, an embodiment is shown in which the conditions for a good chip control are considerably improved. In this case, the height of the flank surface, i.e., the level difference H3 between the bearing surface SS and the cutting edge CE, is selected in such a way that the chip CH will carefully meet the flank surface FS in an upper area closest to the bearing surface SS. In such a way, the generation of heat and the tendencies to sticking are reduced, whereby the easy-cutting properties of the turning insert are maintained. Not only the fact that the chip hits the flank surface FS by a moderate force, but also the fact that the distance between the cutting edge CE and the point of impact of the chip against the flank surface FS is greater than in FIG. 3 contributes to the moderate heat generation, whereby the temperature in the hot chip will have time to decrease further. When the level difference between the bearing surface SS and the cutting edge CE is selected in an optimal way, as shown in FIG. 4, good chip control is accordingly created.
A great difference between a cutting edge having a positive cutting geometry according to the above and a cutting edge having a negative cutting geometry is that the first-mentioned one lifts out the chip by being wedged in between the same and the generated surface, while the last-mentioned one pushes the chip in front of itself while shearing off the same. Generally, positive cutting edges will therefore be more easy-cutting than negative ones and produce chips having greater radii of curvature than the last-mentioned ones.
Within the field of turning, it is often desirable to be able to use one and the same turning insert for rough, medium and fine turning, while attaining good chip control irrespective of the cutting depth in question. For this reason, a number of different turning inserts of the type initially mentioned have been developed, i.e., turning inserts that include a primary chip-guiding flank surface situated behind the individual nose edge as well as two secondary flank surfaces situated inside the two main edges that converge into the nose edge. Examples of such turning inserts are documented in U.S. Pat. No. 5,372,463, U.S. Pat. No. 5,743,681, and U.S. Pat. No. 7,374,372.
In spite of all development attempts, however, the previously known turning inserts have mediocre versatility in respect of the ability to provide good chip control under all the varying operation conditions possible. Accordingly, certain turning inserts may give acceptable results when the cutting depth is small and the feed moderate (=thin chip), but poor results when the cutting depth as well as the feed are increased (=thicker chip). Other turning inserts are suitable for rough turning at large cutting depths and large feed, but not for fine turning. This lack of versatility becomes particularly annoying when the cutting depth varies during one and the same working operation.
In the turning insert according to U.S. Pat. No. 5,372,463, the primary flank surface is included in an utmost narrow, elongate tongue, which extends along the bisector between two main edges. The smallness of the primary flank surface means that thin chips at small cutting depths risk passing the flank surface without hitting the same. The turning insert according to U.S. Pat. No. 5,743,681 includes a primary flank surface, which per se is sufficiently wide for the chip to hit the same with certainty. With the purpose of further improving this certainty, the flank surface is made with a concave shape. However, as soon as the cutting depth becomes considerably greater than the radius of the nose (=increasing chip width), one of the ends of the primary flank surface will disturb the chip formation by partially applying forces to the chip, which tends to deform the same too abruptly. Nor does the turning insert according to U.S. Pat. No. 5,372,463, which is based on the use of a bean-like knob behind the nose edge, provide universally good chip control. Even if a thin and slender chip perchance would be captured in the central groove running in the length extension of the knob, the rather high side surfaces of the knob will disturb the chip formation as soon as the cutting depth increases.